Due to the pressure from environment protection and energy source cost, the using proportion of the natural gas, as a kind of primary energy, in various fields of the society is gradually promoted, and the market requirement for natural gas is also rapidly increased. The traditional mode of transportation and supply by pipeline still remains a mainstream, but due to the limitation from gas-feed conditions and consumer distribution, a large part of energy resources cannot be transported by pipeline in long distance, so we have to select a liquefying mode, i.e., we transform the methane gas to a liquid and then use a flexible mode of transportation to transport the liquid to user terminals. The volume of liquefied natural gas (LNG) corresponds to 1/625 of volume of the same amount of gaseous natural gas, by liquefaction, the cost of storage and transportation can be reduced, and the combustion value of per unit of volume can also be increased.
In a industrial-scale apparatus to obtain liquefied natural gas (LNG) from a methane-rich gas mixture, before performing a cryogenic liquefaction of the gas mixture, the acidic gas components, the water and the high-carbon hydrocarbons (≧C6) etc. contained in the gas mixture must be removed to a degree required by liquefaction, so the stable safety operation of the liquefaction separation process and the apparatus can be ensured. As to the operating conditions of several LNG apparatuses put into service in China, the front-end purifying section of gas mixture usually utilizes an acidic gas removing unit, a drying and adsorbing unit, and a heavy hydrocarbon removing unit, etc. to remove impurity components in successive steps, the drawbacks of these operating conditions consist in that the equipment investment is higher and the energy consumption of the system under normal operating mode is also higher.